1. Field of the Invention
The invention disclosed in this specification and the like relates to a semiconductor device and a method for manufacturing the semiconductor device.
In this specification and the like, a semiconductor device refers to all types of devices that can function by utilizing semiconductor characteristics, and includes in its category an electro-optical device, an image display device, a semiconductor circuit, and an electronic device.
2. Description of the Related Art
Attention has been drawn to a technique of forming transistors with use of semiconductor thin films formed over a substrate having an insulating surface. These transistors are widely used for electronic devices such as an integrated circuit (IC) and an image display device (also simply referred to as a display device). Although a silicon-based semiconductor material is a common material for a semiconductor thin film applicable to a transistor, an oxide semiconductor has been attracting attention as another material.
For example, a technique of forming a transistor with use of an oxide semiconductor such as zinc oxide or an In—Ga—Zn-based oxide semiconductor is disclosed (see Patent Documents 1 and 2).
Another technique is also disclosed: oxide semiconductor layers with different electron affinities (or lower levels of the conduction bands) are stacked to increase the carrier mobility of a transistor (see Patent Documents 3 and 4).